Deep drawing steel sheet and method for producing the same



United States Patent 3,335,036 DEEP DRAWHNG STEEL SHEET AND METHOD FORPRODUCING THE SAME Hiroshi Yoshida and Yoshio Nakazato, Chiba-shi,Japan, assignors to Kawasaki Steel Corporation, Fulriai-ku, Kobe-shi,Hyogo-ken, Japan, a corporation of Japan No Drawing. Filed Jan. 19,1065, Ser. No. 426,667 Claims priority, application Japan, Jan. 25,1964, 39/ 3,412 18 Claims. (Cl. 148-2) ABSTRACT OF THE DISCLOSURE A deepdrawing rimmed steel sheet and method of manufacture wherein the sheetconsists of up to .02% carbon, 0.20-O.60% manganese, 0.0030.050%selenium, the balance iron and incidental impurities and wherein thesheet is produced by subjecting a rimmed steel sheet containing0.020.l0% carbon along With the requisite manganese and selenium to hotand cold rolling to produce the sheet and thereafter decarburizing toreduce the carbon content to less than 0.02%.

The present invention relates to a cold rolled steel sheet which hassuperior deep drawing qualities, excellent nonaging properties, andwhich is not subject to surface imperfections by fabrication and to amethod for producmg same.

Heretofore, aluminum killed steels adapted for both deep drawability andnon-aging have been extensively employed for use in automobile bodysheet, various kinds of machine parts, electrical appliances, homeappliances, furniture, etc. However, the aluminum killed steels areinferior to rimmed ones in the yield of ingots as well as in surfacefinish and their production costs are high. The conventional rimmedsteels are of low cost and of good appearance but rimmed steels have notbeen utilized in the past for the above purposes because of their ratherpoor deep drawing as well as non-aging properties.

This invention contemplates to overcome the above disadvantages ofrimmed steels, the principal object of this invention being to providean improved rimmed steel sheet and a method of producing the same whichis free from surface defects, of high yield as well as of low cost, andwith excellent deep drawing as well as nonaging properties.

This invention is directed specifically to a deep drawing rimmed steelsheet having a composition consisting of up to 0.02% carbon, 0.20-0.60%manganese, 0.003- 0.050% selenium, the balance iron and incidentalimpurities, the physical properties of which are:

(1) A crystal grain size number of 8.010.0 according to A.S.T.M.standards (American Society for Testing Materials) (2) A C.C.V. (conicalcup value) of less than 37.3 when the sheet thickness is 0.8 mm.; and,

(3) a (111) crystal aggregate structure.

In addition this steel may contain 0.010-0.040% phosplfatorus, thepurpose for which will be described hereina er.

A method for producing this improved cold rolled steel sheet firstcomprises adding 0.0030.050% selenium to a molten steel containing0.020.l0% carbon and 0.20- 0.60% manganese to produce a rimmed steel ofthe above specified composition. When phosphorus is also to be addedboth selenium and phosphorus are incorporated in the molten steelcontaining 0.020.10% carbon, 0.20- 0.60% manganese, and less than 0.010%phosphorus to produce a rimmed steel containing 0.0030.050% selenium and0.010-0.040% phosphorus. After the additions are completed this steel issubjected to a series of hot and cold rolling procedures and then to adecarburizing anneal at a temperature of 600800 C. to reduce the carboncontent to less than 0.02%.

According to a recent investigations there exists a close relationbetween deep drawability of steel sheet and crystal aggregate structure.It has been found that the crystal aggregate structure in which the(111) plane of the iron crystal is arranged in the orientation parallelto the rolling plane is the most favorable one for obtaining deepdrawability in steel sheet.

However, a rimmed steel sheet of such crystal aggregate structure as theabove has never been satisfactorily produced by the conventional methodfor obtaining a cold rolled steel sheet and, therefor, a good deepdrawability has not been obtained in such sheets. It has been discoveredthat when the steel sheet is subjected to cold rolling and adecarburizing anneal, the content of carbon deleterious to the deepdrawability of the steel sheet is reduced to a minimum with the (111)iron crystal aggregate structure being developed by the growth ofcrystal grains during decarburization. This (111) iron crystal aggregatestructure will then be the main orientation of the aggregate structureof the cold rolled and annealed steel sheet, and the structure willdevelop a plastic anisotropy favorable for the deep drawing workingprocess. Thus, this method can give a rimmed steel a deep drawingquality.

However, the above decarburizing anneal tends to develop the growth ofextraordinary crystal grains, and a steel sheet having a coarse crystalgrain will tend to produce surface defects when subjected to press Work.To inhibit surface defects the crystal grain size number should be inthe range of 8.0-l0.0 as specified by the A.S.T.M.

In order to control the growth of crystal grains, we first considered ifwe could control the anneal hour so as to stop the decarburizing annealprior to the occurrence of extraordinary growth of crystal grains but wefound that such was not feasible for an actual operation since itmaintains the carbon content of a steel sheet product at a relativelyhigh level with a resultant considerable non-uniformity in productquality. Secondly, we considered if we could adopt a relatively lowtemperature for anneal and soak a steel product in it, but by thismethod, although we were effective to a certain degree in inhibiting theextraordinary growth of crystal grains, the decarburizing reaction didnot proceed in a satisfactory manner to enhance deep drawability. It wasthen considered that the reaction in which an undesirable extra- 3ordinary growth of crystal grains takes place is caused by the removalof carbon and its compounds which function to inhibit the migration ofgrain boundary, and therefore, thirdly, we decided to add anotherelement which acts to inhibit the migration of grain boundary. Oxygen,selenium, and sulfur of the Group VII; of the Periodic Table are knownto be such elements to provide the above desired action. Of theseelements oxygen is the cause of non-metallic inclusions whichdeteriorate stretchability and sulfur forms a sulfide unfavorable formetal working. However selenium facilitates grain control in the courseof decarburization, never deteriorates working, and gives an excellentdeep drawability. Further, in comparison with the aluminum in thealuminum killed steep which imparts an ill effect toelongationworka-bility, selenium provides a good stretchability.

In view of our extensive study we have discovered that the carboncontent of the steel ingot produced in the steel making step is requiredto be in the range of 0.020.10% to obtain the desired final productquality. If the carbon content of the steel is lower than the lowerlimit of the above range, the oxygen content thereof increases so muchthat the steel quality is deteriorated by the presence of non-metallicinclusions. If the carbon content is higher than the up er limit anuneconomical extended period of time is required for carrying out adecarburizing anneal described hereafter.

Manganese may be included in an amount of 0.20- 0.60%, which is a normalamount for conventional rimmed steel, Further, as pointed out above,phosphorus may be incorporated in the steel along with the selenium butwhere there is a phosphorus addition the molten steel should be fullydephosphorized to less than 0.010% phosphorus in the steel making step.Where a phosphorus addition is made the final steel product shouldcontain between 0.010 and 0.040% of this element.

The above content of phosphorus is effective not only for thedevelopment of the (111) crystal grain aggregate structure in the steelsheet, but also to inhibit the decrease of tensile strength. Further itimparts a superior deep drawing quality to the steel sheet.

If the content of phosphorus is less than the lower limit, the effect ofphosphorus is hardly recognized while if it is more than the upperlimit, the stretchability .of sheet material is diminished.

The steel sheet with both added elements, phosphorus and selenium, has abetter deep drawability than a sheet added with selenium only asdescribed hereinafter.

It has further been discovered that a favorable effect of phosphorus onthe deep drawability of steel sheet is noticeable only when it is addedto the molten steel which has already been dephosphorized in the steelmaking step. Phosphorus originally present in the raw material will notso function. A theoretical explanation of the above phenomenon is notknown but, as shown in the tables, a clear difference in deepdrawability is recog nized between the steel with added selenium (Nos.5-9) and those with both selenium and phosphorus (Nos. 13).

As pointed out above, a steel containing 0.020.10% carbon and O.200.60%manganese is produced by either converter, open hearth furnace orelectric furnace and then either selenium or selenium and phosphorus areadded to either ladle or mold to obtain a rimmed steel ingot. N0deoxidizing treatment whatsoever is applied.

In this invention the amounts of other impurities than the specifiedelements should be reduced to as low quantities as possible.

The steel ingot thus produced is subjected to a series of conventionalsteps, such as, slabbing, hot rolling, pickling, and cold rolling. Thenthe cold rolled steel sheet is subjected to a decarburizing anneal toreduce the carbon content of the steel sheet to less than 0.02%,preferably less than 0.01%. With a considerable decrease of carbon the(111) crystal grain aggregate structure develops gradually whereby deepdrawability and plastic anisotropy are improved. The type of annealfurnace is not particularly critical but an open coil type anneal ismost desirable from the viewpoint of economy as well as product quality.An anneal temperature is preferred to be in the range of 600-800 C. Ifthe anneal temperature is too low, the time for decarburization willadversely efiect deep drawability. If it is too high an extraordinarygrowth of crystal grain will take place so that it will be diflicult toobtain a product of uniform quality. Preferably the anneal temperatureis in the range of 650750 C. in order to obtain superior deepdrawability with the crystal grain under control.

The composition of the atmosphere gas for the anneal may be any of theknown decarburizing compositions. Further a combination atmosphere maybe utilized. Satisfactory atmospheres include ones consisting ofhydrogen containing a small amount of water. Also the atmosphere maycontain nitrogen which is desirable from an economic standpoint.

EXAMPLE follows:

Percent C 0.06 Si Tr. Mn 0.07 P 0.008 S 0.016

Then ferrophosphorus and ferromanganese alloys were added to the moltensteel which had been poured into the ladle whereby a steel of thefollowing composition was produced:

Percent C 0.08 Si Tr. Mn 0.34 P 0.020 S 0.014

Thereafter the molten steel was poured into an ingot mold while, at thesame time, 670 g. metallic selenium was added and a 10 t. rimmed steelingot was obtained. This steel ingot was heated in a soaking pituniformly to about 1300 C. and then subjected to slabbing to produce aslab of mm. thickness. This slab was hot rolled in a hot strip mill tomake a hot strip coil of 2.8 mm. in thickness. Then, the hot coil waspickled and cold rolled in a cold strip mill to obtain a cold strip coilof 0.8 mm. in thickness. Thereafter, the cold strip coil was subjectedto a decarburizing anneal. The annealing furnace was an open coil typein which an atmosphere gas consisting of a mixture of hydrogen, nitrogenand steam (H :N ==3: 1) was employed. The cold strip coil was subjectedto the decarburizing anneal for 20' hours at the temperature of 710 C.Lastly, the cold coil was subjected to a temperrolling with a slight(1.0%) reduction to produce a smooth surface thereon.

The following tables show chemical analysis (Table I) and physicalproperties (Table II) of steel products in which various amounts ofadded selenium and phosphorus and various anneal temperatures areillustrated. The steel sheets marked in Table I fall within the scope ofthis invention. For comparison, the properties of representative rimmedsteel and aluminum killed steel are also listed in these tables.

TABLE I.GHEMIOAL ANALYSIS OF TEST STEELS Composition (Percent) No. SteelSi S Al N Se Mn P Commercial Rimmed SteeL... .043 Tr .34 .011 .020 Tr.0018 Al-killed Steel .040 Tr .35 .008 .016 .035 .0065 De-C, De-N RimmedStee .006 Tr .34 .008 .016 TI .0008 De-C, De-N Rimmed Steel. .015 Tr .33.007 .018 Tr .0010 Se-contg. De C, De-N Rim. St .006 Tr .36 .009 .020 Tr.0006 .005 Se-con. De-C, De-N Rim. St .008 Tr .36 .009 .015 Tr .0008.010 Se-con. De C, De-N Rim. St .006 Tr 34 .010 016 Tr .0009 030 Se-con.De-C, De-N Rim. St .007 Tr .35 .008 .017 Tr .0009 .003 Se-con. De-C,De-N Rim. St .006 Tr .38 .010 .018 Tr .0009 .028 P-, Se-con. De-C, De-NRim. St .006 Tr .34 .020 .019 Tr .0007 .005 P-, Se-con. De-C, De-N Rim.St 007 Tr .36 .020 018 Tr .0006 .005 P-, Se-con. De-C, De-N Rim. Sh.-..007 Tr .33 .015 .020 Tr .0009 .025 P-, Se-con. De-C, De-N Rim. St .006Tr .35 .018 .017 Tr .0008 .003

TABLE II.PHYSIOAL PROPERTIES OF STEEL'PRODUCTS A.S.T.M. Y.P. T.S. ElY.E. C.C.V Er. No. Grain (Kg/mm?) (Kg/mini (Percent) (Percent) (111111.)(mm.) R value Size No.

1 Drawn through.

In Table II the crystal grain size specified by A.S.T.M. 40

is obtained by summing up the number of crystal grains per square inchby means of a 100 magnification microphotograph.

Tensile strength test is conducted by the use of J.I.S. (JapaneseIndustrial Standard) No. 5 test specimen.

Erichsen test is a cupping test, using a tool with a spherical end of 20mm. diameter to deform the test specimen which is held between annularjaws of 27 mm. internal diameter. The test sheet, which is 3% inchessquare, is first clamped between the jaws to measure the thickness; thejaws are then moved apart by 0.05 mm. and clamped in that position toallow metal to be drawn into the cup as the test progresses. The tool ispressed into the metal until a fracture appears in the cup, and thedepth of the cup at fracture is taken as a measure of the ductility ofthe metal.

With reference to the direct test method for the deep dra'wability ofsteel sheet, the conical cup test has been adopted. This test is calledFukuis Cup Test, the details of which are specified in J.I.S. Z2249.When this conical test is conducted on the steel sheet of 0.8 mm. inthickness, a test specimen of a circular disk form is deep drawn untilit is broken by employing a conical die having a die opening angle of 60and the average value of the maximum and minimum rim diameters of thefractured test specimen are determined as a conical cup value (C.C.V.).Therefore, it follows that the smaller C.C.V., the better deepdrawability. For proper drawability it has been determined that theC.C.V. of the steel sheet of this invention as applied to a sheetthickness of 0.8 mm. is 37.30 or less but C.C.V. depends on sheetthickness, the diameter of a test specimen and the size of a die. Thuswhen the conical cup test is applied as specified in J.I.S. Z-2249 thesteel sheets of this invention have the following 'values'in relation tothe sheet thickness:

C.C.V. of the sheet of this invention 0.6 Less than 26.40

Sheet thickness, mm.:

R value shows whether deep drawability is good or not and can'beobtained by measuring the deformation of a tensile test specimen in thethickness direction and that of the same specimen in the widthdirection. The larger R value the smaller the deformation of sheet inthe tensile deformation which shows greater drawability.

In the tables N0. 1 refers to a known rimmed steel in which the crystalgrain is very fine, but its C.C.V. is 38.48

and its R value 1.14 which shows a poor drawability. No. 2 refers to acommercial aluminum stabilized steel which has a good deep drawabilityfor its fine crystal grain size, and in addition, a good aging property,but

which is of low yield as Well as of poorer surface appearance than thatof the rimmed steel of No. 1. N0. 3 refers to a commercial rimmed steelwhich has been subjected to the decarbnrizing anneal and its deepdrawability is more improved than that of No. 1 but its crystal grain isnoticeably coarsened. No. 4 refers to the same steel as No. 3 butsubjected to a lower anneal temperature, up to 680 C., so as to preventthe crystal grain size from coarsening but, owing to the insufficientanneal, inferior deep drawability results. Nos. 5-13 refer to the steelsheet of this invention. Nos. 5, 6 and 7 refer to the rimmed steel sheetcontaining 0.0030.050% selenium subjected to the decarbnrizing anneal atthe temperature of 710 C., in which the crystal grain size issufficiently fine, and the deep drawability is rather good, its C.C.V.being about 37. No. 8 refers to an improved steel sheet subjected to theanneal temperature of 690 C. in which an appropriate crystal grain sizeis developed by the addition of 0.003% selenium. No. 9 refers to a steelsheet of this invention in which the anneal temperature of 740 C. whichis somewhat high but coarsening of crystal grain is prevented by adding0.028% which is relatively high. Nos. 10, 11 and 12 refer to the rimmedsteel containing 0.0100.040% phosphorus in addition to selenium andwherein the steel is subjected to a decarburizing anneal at thetemperature of 710 C., in which the crystal grain is suitably fine, andits deep drawability is more improved than those of Nos. 5, 6 and 7containing selenium only, and further, in which the decrease of tensilestrength is inhibited. No. 13 refers to the steel sheet contaning 0.003%selenium and 0.018% phosphorus subjected to the anneal at a temperatureof 690 C. in which an 8.6 A.S.T.M. crystal grain size is obtained due tothe slight decrease of selenium content.

Elongation (EL) and stretchability (EL) of the steel sheet in accordancewith the present invention are the same or better than those of thealuminum killed steel.

The test results of yield point-elongation (Y.E.), one of the criteriafor non-aging, show that the steel sheet of this invention has asufficiently low Y.E. value, which is evidenced by the excellentnon-aging property. It is believed that this is caused by the removal ofnitrogen from the steel sheet by the action of hydrogen in the course ofthe decarburizing anneal step. Accordingly, the removal of carbon aswell as nitrogen from the steel sheet of this invention is advantageousfor making the steel sheet non-aging. To this end, it is desirable toincrease the hydrogen partial pressure and decrease the nitrogen partialpressure in the annealing atmosphere.

In the example described (No. with the atmosphere gas consisting of theratio H :N =3:1, the resultant steel sheet had a nitrogen content toabout 00006-00009 Various changes and modifications of this inventioncan be made and, to the extent that such variations incorporate thespirit of this invention, they are intended to be included within thescope of the appended claims.

We claim:

1. A method for producing a deep drawing steel sheet which comprisesforming a rimmed steel ingot containing 0.020.l0% carbon, 0.200.60%manganese, and 0.0030.050% selenium, subjecting said rimmed steel ingotto hot and cold rolling to produce a thin steel sheet, and thendecarburizing said sheet by anneal at a temperature of 600-800" C. toreduce the carbon content to less than 0.02%.

2. A method according to claim 1 wherein the decarburizing annealatmosphere has a low nitrogen partial pressure whereby the nitrogencontent of the steel is reduced.

3. A method according to claim 1 wherein after the final anneal thesteel sheet is temper-rolled to produce a smooth surface.

4. A method according to claim 2 wherein the anneal is of the open coiltype.

5. A method according to claim 1 wherein the decarburizing atmosphere isa mixture of hydrogen, nitrogen and steam.

6. A method according to claim 1 wherein the carbon content is reducedby the anneal to less than 0.01%.

7. A method according to claim 1 wherein the decarburizing anneal iseffective at a temperature of between 650 and 750 C.

8. A method for producing a deep drawing steel sheet which comprisesproviding a rimmed steel ingot containing 0.0030.050% selenium and0.010.040% phosphorus by adding selenium and phosphorus to a moltenrimmed steel containing 0.020.10% carbon, 0.20 0.60% managnese, and upto 0.010% phosphorus, subjecting said rimmed steel ingot to hot rollingand then to cold rolling to produce a thin steel sheet, and thendecarburizing said sheet by an anneal at a temperature of 600-800 C. toreduce the carbon content to less than 0.02%.

9. A method according to claim 8 wherein the decarburizing annealatmosphere has a low nitrogen partial pressure whereby the nitrogencontent of the steel is reduced.

10. A method according to claim 8 wherein after the anneal the steelsheet is temper-rolled to produce a smooth surface.

11. A method according to claim 9 wherein the anneal is of the open coiltype.

12. A method according to claim 8 wherein the decarburizing atmosphereis a mixture of hydrogen, nitrogen and steam.

13. A method according to claim 8 wherein the carbon content is reducedby the anneal to less than 0.01%.

14. A method according to claim 8 wherein the decarburizing anneal isefiected at a temperature between 650 and 750 C.

15. A deep drawing rimmed steel sheet consisting of up to 0.02% carbon,0.200.60% manganese, 0.003- 0.050% selenium, the balance iron andincidental impurities, said steel sheet characterized by having anA.S.T.M. crystal grain size number of 8.010.0, a C.C.V. of less than37.3 for 0.8 mm. thickness and containing predominantly a (111) crystalaggregate structure.

16. A deep drawing sheet in accordance with claim 15 wherein the carboncontent is less than 0.01

17. A deep drawing rimmed steel sheet consisting of up to 0.02% carbon,0.200.60% manganese, 0.003- 0.050% selenium, 0.010-0.040% phosphorus,the balance iron and incidental impurities, said steel sheet beingcharacterized by having an A.S.T.M. crystal grain size number of 80-100,a C.C.V. of less than 37.3 for 0.8 mm. thickness and containingpredominantly a (111) crystal aggregate structure.

18. A deep drawing sheet in accordance with claim 17 wherein the carboncontent is less than 0.01%.

References Cited UNITED STATES PATENTS 2,009,713 7/1935 Palmer l232,009,714 7/1935 Palmer 75-123 2,258,604 10/1941 Gagnebin 75-1232,316,948 4/1943 Gagnebin 75123 3,239,388 3/1966 Sasaki l4812 CHARLES N.LOVELL, Primary Examiner.

8. A METHOD FOR PRODUCING A DEEP DRAWING STEEL SHEET WHICH COMPRISESPROVIDING A RIMMED STEEL INGOT CONTAINING 0.003-0.050% SELENIUM AND0.01-0.040% PHOSPHORUS BY ADDING SELENIUM AND PHOSPHORUS TO A MOLTENRIMMED STEEL CONTAINING 0.02-0.10% CARBON, 0.200.60% MANAGNESE, AND UPTO 0.010% PHOSPHORUS, SUBJECTING SAID RIMMED STEEL INGOT TO HOT ROLLINGAND THEN TO COLD ROLLING TO PRODUCE A THIN STEEL SHEET, AND THENDECARBURIZING SAID SHEET BY AN ANNEAL AT A TEMPERATURE OF 600-800*C. TOREDUCE THE CARBON CONTENT TO LESS THAN 0.02%.